Heat Units in plant physiology and the importance of Growing Degree days
Lethal genes
1. LETHAL GENES
SUBMITTED BY;
SATISH KANNA . M
1ST MSc. Botany
Bharathidasan University
Tiruchirappalli – 620 024
SUBMITTED TO;
Dr. T. SENTHIL KUMAR M.Sc.,M.Phil.,Ph.D
Department of Botany
Bharathidasan University
Tiruchirappalli – 620 024
2. TABLE OF CONTENTS
Defination ,History, Observers and their Observations a quick
overview.
What Are Lethal Genes ?
01
An overview of major types of lethal genes or lethal alleles
Types of Lethal Genes02
An overview
Lethal gene in mouse04
An overview.
Lethal gene in Snapdragon03
4. WHAT ARE LETHAL GENES ?
Lethal Gene - a gene that leads to the death of an individual.
• Certain genes are absolutely essential for survival. Mutation in these genes creates
lethal allele / lethal genes.
• It can be either dominant or recessive in nature or alleles that cause an organism to die
only when present in homozygous condition are called lethal alleles. The gene involved
is considered as an essential gene.
• Essential genes are those indispensable for the survival of an organism, and therefore
are considered a foundation of life.
• Lethal alleles are dominant or recessive.
• Fully dominant lethal allele kills organism in both homozygous and heterozygous
condition .
• Certain lethal alleles kills organisms in homozygous condition only.
5. 1905
E. Baur (1907) observed lethal
gene in Snapdragon
(Antirrhinum) and found that it
is characterized by variegated
leaves
1910
HISTORY
1907
A French geneticist L. Cuenot
(1905) reported lethal gene on
the inheritance of mouse body
colour.
W. E. Castle and C. C.
Little confirmed Cuénot's work and
the first documented example of a
recessive lethal allele was recorded.
6. 01
02
Discovered the concept of
lethal gene first in animals.
Lucien Cuenot
(1905)
Observed the lethal gene first
in plants .
E. Baur (1907)
Lethal gene in mouse body
coat(colour)
Lethal gene in Snapdragon
(Antirrhinum)
E. Baur (1907) observed lethal gene in Snapdragon
(Antirrhinum) and found that it is characterized by
variegated leaves. The “golden” variety on selfing
gives rise to 2 types of offsprings, golden and green in
the ratio of 2:1 instead of 3: 1. The golden ones are
heterozygous and the green ones breed true being
recessive homozygous.
A French geneticist L. Cuenot (1905) reported on the
inheritance of mouse body colour. He found that
“yellow” body colour was dominant over normal
“brown” colour and was governed by single gene
“Y”. It was observed that yellow mice could never
be obtained in homozygous condition.
7. TYPES OF LETHALALLELES
Lethal alleles which result in early
death of an organism ,during
embryogenesis .
1) EARLY ONSET
Lethal alleles which kill organism
under certain environment conditions
only .
For example, a temperature sensitive
lethal allele may kill organism at high
temperature, but not at low
temperature.
3) CONDITIONAL
Lethal genes which have delayed
effects so that the organism can live
for some time but eventually succumb
to the disease.
2) LATE ONSET
Lethal alleles which kills only some
individuals in the population but not
all.
4) SEMI LETHAL
4 MAJOR
TYPES
8. TYPES OF LETHAL ALLELES
Dominant Lethal Genes
• Scientists studying the fruit fly observed that pairwise combinations of some mutant alleles
were not viable, whereas singly, the same mutant alleles did not cause death (Boone et al.,
2007).
• In other words, some mutations are only lethal when paired with a second mutation. These
genes are called synthetic lethal genes.
• When the functions of the two affected genes are not fully understood, scientists can create
and study synthetic lethal mutants and their phenotypes to identify a gene's function.
• Dominant lethal genes are expressed in both homozygotes and heterozygotes.
• These are rarely detected due to their rapid elimination from populations.
• Example ; i)coat colour inheritance of mouse [L. Cuenot (1905) ]
ii) Huntington’s disease in Human Beings.
Synthetic Lethal Genes
9. Lethal gene in Snapdragon (Antirrhinum majus)
• In 1907, Edwin Baur began his work with the snapdragon
plant Antirrhinum and found that it is characterized by
variegated leaves(characterized the condition aurea), in which
plants had golden instead of green leaves.
• When two aurea snapdragon plants were crossed, Baur
observed a 2:1 ratio of golden seedlings to green seedlings.
• Homozygous aurea plants lacked normal chlorophyll
development and died either during the embryonic stage or
when the plant seedlings were two to three days old.
• Examples of human diseases :- cystic fibrosis and sickle cell
anemia .
10. Lethal gene in Mouse
• A French geneticist L. Cuenot (1905) reported on the inheritance
of mouse body colour. He found that “yellow” body colour was
dominant over normal “brown” colour and was governed by
single gene “Y”. It was observed that yellow mice could never be
obtained in homozygous condition.
• When yellow coated mice was crossed with another yellow
coated mice, segregation for yellow and brown body colour was
obtained in 2:1 ratio. The brown individuals were pure and
homozygous where as yellow individuals were heterozygous.
These results may be explained on the assumption that the
dominant allele for yellow body colour is lethal in homozygous
condition.
